In refractive index imaging, a pattern of varying refractive indices is created within the material used to record the image. This pattern is commonly referred to as a phase hologram. When light is subsequently transmitted through, or directed onto the surface of the recording medium, the phase of the light is modulated by the pattern of refractive indices.
Early developments in the field of refractive index imaging are described in a number of basic references, including "Photography by Laser" by E. N. Leith and J. Upatnieks appearing in Scientific American 212, No. 6, June 1965. A useful discussion of holography is presented in "Holography" by C. C. Guest, in Encyclopedia of Physical Science and Technology, Vol. 6, pp. 507-519, R. A. Meyers, Ed., Academic Press, Orlando, FL, 1987. In brief, the object to be imaged is illuminated with coherent light (e.g., from a laser), and a light sensitive recording medium (e.g., a photographic plate), is positioned to receive light reflected from the object. Each point on the object reflects light to the entire recording medium, and each point on the medium receives light from the entire object. This beam of reflected light is known as the object beam. At the same time, a portion of the coherent light is directed by a mirror directly to the medium, bypassing the object. This beam is known as the reference beam. What is recorded on the recording medium is the interference pattern that results from the interaction of the reference beam and the object beam impinging on the medium. When the processed recording medium is subsequently illuminated and observed at the appropriate angle, the light from the illuminating source is diffracted by the hologram to reproduce the wave-front that originally reached the medium from the object. Thus, the hologram resembles a window through which the virtual image of the object is observed in full three-dimensional form, complete with parallax.
Holograms formed by allowing the reference and object beams to enter the recording medium from opposite sides, so that they are traveling in approximately opposite directions, are known as "reflection holograms". Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are, approximately, planes parallel to the plane of the recording medium. When the hologram is played back these fringes act as mirrors reflecting incident light back to the viewer. Hence, the hologram is viewed in reflection. Since the wavelength sensitivity of this type of hologram is very high, white light may be used for reconstruction.
Reflection holograms may be produced by an in-line or on-axis method wherein the beam of coherent radiation is projected through the recording medium onto an object therebehind. In this instance, the reflected object beam returns and intersects with the projected beam in the plane of the recording medium to form fringes substantially parallel to the plane of the medium. Reflection holograms also may be produced by an off-axis method wherein a reference beam is projected on one side of the recording medium and an object beam is projected on the reverse side of the medium. In this instance the object beam is formed by illuminating the object with coherent radiation which does not pass through the recording medium. Rather, the original beam of coherent radiation is split into two portions, one portion being projected on the medium and the other portion being projected on the object behind the medium. Reflection holograms produced by an off-axis process are disclosed in U.S. Pat. No. 3,532,406 to Hartman.
A holographic mirror is the simplest possible reflection hologram. It is the hologram of two coherent plane waves intersecting in a recording medium from substantially opposite directions. It can be created by splitting a single laser beam and recombining the beams at the recording medium, or the unsplit laser beam can be projected through the medium onto a plane mirror therebehind. A set of uniformly spaced fringes is thereby formed, with the fringes oriented parallel to the bisector of the obtuse angle between the two projected beams and having an intensity that is a sin.sup.2 function. If the obtuse angle is 180.degree. and the projected beams are normal to the plane of the medium, the fringes will be parallel to the plane of the medium. If the obtuse angle is less than 180.degree. or if both beams are not normal to the plane of the medium, reflective fringes will be formed which will be slanted at an acute angle relative to the plane of the medium. The holographic mirror can be characterized by its reflection efficiency (i.e., by the percent of incident radiation which is reflected), by its refractive index modulation, and by the spectral bandwidth and character of the reflected radiation.
The substantially horizontal fringes which form reflection holograms are much more difficult to record than the perpendicular fringes which form transmission holograms for two reasons. The first reason is the need for higher resolution (i.e., the need for more fringes per unit length, and thus a smaller fringe spacing). Reflection holograms, operating at a given wavelength, require about 3.times. to 6.times. more fringes per unit length than do transmission holograms at the same wavelength. The second reason is the sensitivity of horizontal fringes to shrinkage of the recording medium. Any shrinkage of the recording medium during exposure will tend to wash out the fringes and, if severe, will prevent a hologram from being formed. This is in contrast to the case with transmission holograms, where shrinkage has little or no effect if the fringes are perpendicular to the plane of the medium, and only relatively minor image distortion is produced if the transmission fringes are slanted less than 45.degree. from perpendicular.
A variety of materials have been used to record volume holograms. Among the more important are: silver halide emulsions, hardened dichromated gelatin, photorefractives, ferroelectric crystals, photopolymers, photochromics and photodichroics. Characteristics of these materials are given in Volume Holography and Volume Gratings, Academic Press, New York, 1981 Chapter 10, pp. 254-304 by L. Solymar and D. J. Cook.
Dichromated gelatin is currently the material of choice for making holograms due to its high values of refractive index modulation (i.e., high diffraction efficiency, wide bandwidth response,"). However, dichromated gelatin has poor shelf life and requires wet processing after the material has been imaged to contain a hologram. Due to its poor shelf life, the material must be freshly prepared shortly before imaging or prehardened gelatin must be used, which reduces image efficiency. Wet processing introduces an additional step in preparation of the hologram, and causes dimensional changes in the material as it swells, then shrinks, during processing. These dimensional changes affect spacing of the interference fringes. Thus, it is difficult and time consuming to reproducibly make high quality holograms with dichromated gelatin.
Substantially solid, photopolymer films have heretofore been proposed for use in making holograms. U.S. Pat. No. 3,658,526 to Haugh, for instance, discloses preparation of stable, high resolution holograms from solid, photopolymerizable films by a single step process wherein a permanent refractive index image is obtained by a single exposure to a coherent light source bearing holographic information. The holographic image thus formed is not destroyed by subsequent uniform exposure to light, but rather is fixed or enhanced.
Despite the many advantages of the materials proposed by Haugh, they offer only limited viewing response to visible radiation and application has been limited to transmission holograms where the holographic image is viewed by light transmitted through the imaged material. Moreover, the materials disclosed in Haugh have little or no reflection efficiency when the material is imaged to form a reflection hologram. Thus, there continues to be a need for improved materials for use in preparing holograms in general, and reflection holograms in particular.